1. Field of the Invention
The present invention relates to a position measurement apparatus and a position measuring method for measuring a position on the surface of a measurement object by capturing an image of the surface of the measurement object.
2. Description of the Related Art
For measuring the length of a measurement object (in the present specification, this is referred to as the total length of the measurement object, the size of a part thereof, or the displacement, etc., of the measurement object), an image capturing position measurement apparatus has been used. Such a position measurement apparatus captures an optical image irradiated from the surface of the measurement object using an image sensor made up of a CCD, CMOS image sensor array or the like, and analyzes the image signal obtained by such image capturing to thereby measure the length of the measurement object.
For example, in Japanese Laid-Open Patent Publication No. 2002-013948 and in Japanese Laid-Open Patent Publication No. 2003-148918, systems are disclosed in which an imaging region with respect to a measurement object is set narrowly, so that displacement of the measurement object can be measured at high resolution and with high accuracy.
However, even when such an image capturing displacement measurement apparatus is used, in the case that the length of a measurement object having a large measurement range is to be measured, because a trade off relationship occurs between the size of the measurement range and spatial resolution, various difficulties in measurement tend to occur. Specifically, when the image capturing magnification ratio is small, although the entire measurement object can be set inside of the imaging region, on the other hand, since the imaging spatial resolution is decreased, sufficient measurement accuracy cannot be obtained.
Further, when the image capturing magnification ratio is made larger, although good measurement accuracy can be obtained because imaging spatial resolution is improved, the entire measurement object cannot be set inside of the imaging region. Stated otherwise, this is because the imaging region and spatial resolution are finite since there is a limit imposed on the capacity of the image sensor and the configuration of the apparatus.
On the other hand, although it can be considered to increase the number of pixels that make up the image sensor and to provide a configuration having a large imaging region, this will make the position measurement apparatus large in scale and drastically increase manufacturing costs thereof.